1. Field of the Invention
The present invention relates to a radiation curable powder coating composition which crosslinks to form a lightfast and weather-stable film.
2. Discussion of the Background
Thermally crosslinkable powder coating materials are known and are much used in the paint processing industry.
For example, DE-C 27 35 497 describes PU powder coatings having outstanding weathering stability and heat stability. The crosslinkers whose preparation is described in DE-C 27 12 931 are composed of xcex5-caprolactam-blocked isophorone diisocyanate containing isocyanurate groups. Also known are polyisocyanates containing urethane, biuret or urea groups, their isocyanate groups again being blocked.
The disadvantage of these systems lies in the elimination of blocking agent during the thermal crosslinking reaction. Since, consequently, the blocking agent may be emitted into the environment, it is necessary on ecological and industrial hygiene grounds to take special precautions to clean the outgoing air and/or to recover the blocking agent. Moreover, the reactivity of the crosslinkers is low. Curing temperatures above 170xc2x0 C. are required.
Both disadvantagesxe2x80x94emission of blocking agent and curing at high temperaturesxe2x80x94can be removed by using powder coating materials which are cured not thermally but instead by means of radiation. Radiation curable powder coating materials of this kind are known and are described in, for example, U.S. Pat. No. 3,485,732, EP 0407826, EP 0636669, WO 99/14254, U.S. Pat. Nos. 3,974,303, 5,639,560 and EP 0934359. Two-component, radiation curable powder coating materials based on an unsaturated polyester and a vinyl ether. The coatings produced from them are unsuitable for outdoor use due to their severe yellowing.
U.S. Pat. No. 3,974,303 describes thermoplastic resins containing from 0.5 to 3.5 polymerizable unsaturated groups per 1,000 g molecular weight. The polyurethanes described (see Example 1), however, are not weather-stable and are of poor flexibility owing to the absence of polyester groups and the low chain length.
U.S. Pat. No. 5,639,560 describes radiation curable powder compositions comprising special crystalline polyesters, additionally containing methacrylic end groups, as binders. These powder compositions optionally comprise ethylenically unsaturated oligomers, including urethane acrylates, in minor amounts, preferably up to 10 percent by weight. The radiation curable powder coating formulations presented in this patent, however, have significant disadvantages. They can be ground only at minus 80xc2x0 C. and, in general, the resultant coatings are either not stable to outdoor weathering or not flexible. A particularly serious disadvantage is that of the very low pendulum hardness (according to Kxc3x6nig) of below 120 s.
EP 0934359 describes powderous radiation curable mixtures of amorphous and crystalline polyesters containing terminal methacrylate groups. The powder coatings produced from them possess a higher glass transition temperature than the crystalline methacrylates of U.S. Pat. No. 5,639,560. However, cooling is still required for their grinding. Moreover, the hardness and adhesion of corresponding coatings are no more than satisfactory.
It is an object of the present invention to provide radiation curable powder coating compositions which are stable when stored at 35xc2x0 C. and which also, after curing of the corresponding films, lead to coatings combining hardness with flexibility and stability to outdoor weathering.
It is another object of the present invention to provide coatings that are highly transparent and coatings that are opaque.
It is yet another object of the present invention to provide the powder coating materials that can be ground with little or no refrigerant.
This and other objects have been achieved by the present invention the first embodiment of which includes a radiation curable powder coating composition, comprising:
I. a binder comprising
A) 60-90% by weight of at least one amorphous urethane acrylate; and
B) 10-40% by weight of at least one crystalline urethane acrylate;
wherein a Tg of a mixture of A) and B) is at least 35xc2x0 C.; and
II. an auxiliary and an additive, excluding an UV initiator.
In another embodiment, the present invention includes a process for producing a radiation curable powder coating composition, comprising:
admixing in a heatable kneading device at a temperature of between 120xc2x0 C. and 130xc2x0 C.
A) 60-90% by weight of at least one amorphous urethane acrylate;
B) 10-40% by weight of at least one crystalline urethane acrylate; and
C) an auxiliary and an additive, excluding an UV initiator;
wherein a Tg of a mixture of A) and B) is at least 35xc2x0 C.
In a further embodiment the present invention includes a coating obtained by the above process.